This invention relates to field emission devices, and more particularly to processes for creating gate and focus ring structures which are self-aligned to emitter tips using chemical mechanical planarization (CMP) and etching techniques.
Flat panel displays have become increasingly important in appliances requiring lightweight portable screens. Currently, such screens generally use electroluminescent or liquid crystal technology. A relatively new technology is the field emission display which uses of a matrix-addressable array of cold cathode emission devices to excite cathodoluminescent material on a screen.
With reference to FIG. 1, a conventional field emission display 10 includes a base plate 12 and a face plate 24 spaced from each other to define a sealed envelope 11 therebetween. The sealed envelope 11 may be evacuated as is conventional in field emission displays.
The base plate 12 may include a substrate 18 of silicon or some other material on which a conductive layer 20 is formed, the conductive layer 20 supporting a plurality of conical emitters 22. Only one emitter 22 has been shown to simplify the discussion. An extraction grid 16 formed of a conducting material is positioned above the substrate 18 by a first insulating layer 26 of dielectric material. Each emitter 22 extends into a respective aperture 31 formed in the extraction grid 16. A focus ring layer 14 is positioned over the extraction grid 16. The focus ring layer 14 is also formed of a conductive material and is spaced from the extraction grid 16 by a second insulating layer 28 of a dielectric material. A plurality of apertures 33 are formed in the focus ring layer 14, each aperture 33 aligned with a respective aperture 31 formed in the extraction grid 16.
The face plate 24 includes a transparent substrate 38 coated with a transparent layer of conductive material 40, such as iridium, forming an anode 36. The anode 36 is, in turn, coated with a layer of cathodoluminescent material 42.
In practice, the emitters 22 (which may be in sets of interconnected emitters) are arranged in columns while individual extraction grids 16 are arranged in rows. An individual emitter 22 can then be selected for electron emission by driving a column of emitters 22 to a relatively low voltage and driving an extraction grid 16 row to a relatively high voltage. Electrons 34 are emitted from the emitter 22 in the energized column of emitters 22 that intersects with the energized extraction grid 16 row.
A relatively high positive voltage on the order of 1000 volts is applied to the anode layer 40. The strong positive voltage attracts the electrons 34 emitted by the emitter 22 so that they pass through the focus ring 14 and strike the cathodoluminescent layer 42. The cathodoluminescent layer 42 then emits light which is visible through the transparent substrate 38.
While the focus ring 50 nominally serves the function of collimating the electron beam 34, the primary purpose of the focus ring layer 14 is to protect the underlying structure from electromagnetic radiation such as soft x-rays and ultraviolet radiation, thus serving as an opaque. Ultraviolet radiation and soft x-rays result from back-scattering from the emitted electrons 34 striking the cathodoluminescent layer 42, resulting in some of the electromagnetic radiation being reflected back toward the back plate 12 from the face plate 24.
The clarity, or resolution, of a field emission display is a function of a number of factors, including emitter tip sharpness, alignment and spacing of the gates, or grid openings 31, which surround the emitter tips 22, pixel size, as well, as cathode-to-gate and cathode-to-screen voltages. Another factor which affects image sharpness is the angle at which the emitted electrons 34 strike the phosphors 42 of the display screen 36.
The distance that the emitted electrons 34 must travel from the base plate 12 to the face plate 24 is typically on the order of several hundred microns. The contrast and brightness of the display are optimized when the emitted electrons 34 impinge on the phosphors 42 located on the cathode luminescent screen 36 or face plate 24, at a substantially 90xc2x0 angle. However, the contrast and brightness of the display are not currently optimized due to the fact that the initial electron trajectories assumes substantially conical patterns having an apex angle of roughly 30xc2x0, which emanates from the emitter tip 22. In addition, the space-charge affect results in coulombic repulsion among emitted electrons 34, which lends to further dispersion within the electron beam 34. Even though the focus rings 50 are normally maintained at ground, they will exert a force on the emitted electrons 34. Since the focus rings 50 are spaced relatively above and outward of the gate structures 30 the force exerted will contribute to the dispersion of the emitted electrons 34.
The current design and positioning of focus ring layer 14 causes several problems. The position of the focus ring 50 which is spaced relatively above the low potential anode or extraction grid 16 with respect to the cathode luminescent panel 36 tends to further disperse the emitted electron beam 34. The current method of fabricating the base plate 12 of the field emission display device 10 requires one CMP step and three etching steps which increases the cost and time required to produce the field emission display. Further, the substantial gap between the extraction grid 16 and the focus ring 50 required by the existing design increases the likelihood of electromagnetic radiation leakage past the opaque.
The present invention overcomes the limitations of the prior art by providing a flat panel display structure having a focus ring which lies in substantially the same plane as the extraction grid. The base plate of the field emission display is manufactured by covering an emitter substrate having emitters tips with a dielectric insulating material to form a first insulating layer, depositing an extraction grid layer over the first insulating layer, etching the extraction grid layer to define a plurality of gate structures, depositing a second insulating layer over the etched structure, depositing a focus ring layer, and chemical-mechanical planarizing the resulting structure to an endpoint at which the emitter tips are at least partially exposed, thus defining self-aligned and in-plane gate and focus ring structures. The structure may then be optionally selectively wet etched to remove portions of the first and second insulating layers for further exposing the emitter tips.
The base plate includes a substrate, a cathode formed on the substrate having an emitter tip, a first insulating layer formed superadjacent the cathode, an extraction grid formed superadjacent the first insulating layer, the extraction grid having a distal surface with respect to the substrate, a focus ring formed superadjacent the extraction grid, the focus ring having a distal surface with respect to the substrate, the distal surface of the extraction grid and the distal surface of the focus ring being substantially planar proximate the emitter tip.
Placement of the focus ring in substantially the same plane as the extraction grid provides a number of benefits over the current design. In plane placement of the focus ring significantly reduces the dispersive effect the focus ring has on emitted electron beam. Use of the in-plane focus ring also permits the number of processing steps to be reduced from three etching steps and one CMP step, to either one or two etching steps and one CMP step, thereby saving substantial time and costs in the manufacturing process. One of the etching steps is performed before the CMP step. The optional second etching step is performed after the CMP step. The in-plane placement of the focus ring also permits a smaller spacing to be used between the focus ring and the gate structure which results in more overlap therebetween, thereby increasing the effectiveness of the focus ring layer as an opaque. The novel structure and process of the present invention also permits identical materials to be used for the extraction grid and the focus ring layer since these layers are no longer required to be selectively etchable with respect to one another. These and other benefits will become apparent to one skilled in the art from reading the detailed description and figures of the exemplary embodiments of the invention which follow.